Cyanines Substituted on the Polymethine Chain: Synthesis, Resulting Properties, and Application Use Cases

Cyanines comprise a diverse group of small-molecule polymethine dyes combining tunable optical properties with high molar absorptivity and fluorescence emission quantum yield, enabling various applications in bioimaging, diagnostics, molecular electronics, photonics, and nonlinear optics. These applications can be facilitated by adjusting the length of their polymethine chain and their functionalization through their end groups or the polymethine chain. Yet, the latter approach remains largely unexplored, with limited information scattered throughout literature. This review focuses on cyanines substituted on their chain, covering their synthesis, properties, and applications and providing an overview of how substituents on their polymethine chain influences their spectroscopic properties, akin to other factors, such as polymethine length and end groups. Lastly, this review illustrates how substituents on the polymethine chain facilitate the application of cyanine dyes in promising research areas.